When you open PowerPoint 2016 for the first time, you’ll see that all slide examples in the Presentation Gallery are in Widescreen aspect ratio with 16:9 proportions. This is in complete contrast to the typical standard slides with 4:3 aspect ratios that you saw in PowerPoint versions older than 2010. For many of us who are living in a world with widescreen displays and projectors, this may be a blessing — but many others may not feel as blessed! Worse, there’s no obvious way for you to change these defaults so that you can always have 4:3 slides rather than 16:9.
Learn about default 16:9 widescreen slide option in PowerPoint 2016 for Windows.